High frequency semiconductor integrated circuit and radio communication system

ABSTRACT

A communication semiconductor integrated circuit has an oscillator circuit forming part of a transmission PLL circuit fabricated on a single semiconductor chip together with an oscillator circuit forming part of a reception PLL circuit and an oscillator circuit for an intermediate frequency. The oscillator circuit for the transmission PLL circuit is configured to be operable in a plurality of bands. The communication semiconductor integrated circuit also comprises a circuit for measuring the oscillating frequency of the oscillator circuit for the transmission PLL circuit, and a storage circuit for storing the result of measurement made by the measuring circuit. A band to be used by the oscillator circuit for the transmission PLL circuit is determined based on values for setting the oscillating frequencies of the oscillator circuit forming part of the reception PLL circuit and the intermediate frequency oscillator circuit, and the result of measurement stored in the storage circuit.

CROSS-REFERENCE TO RELATED APPLICATION

The present application relates to subject matters described in the U.S.patent applications being file based on the United Kingdom PatentApplications No. 0221310.1 filed on Sep. 13, 2002. That U.S. applicationis assigned to the same assignees of the present application.

BACKGROUND OF THE INVENTION

The present invention relates generally to techniques which areeffectively applied to an on-chip voltage controlled oscillator circuit(VCO) that can switch oscillating frequencies, and more particularly, totechniques which are effectively utilized in high frequencysemiconductor integrated circuits and radio communication systems fordemodulating a reception signal and modulating a transmission signal inradio communication apparatuses, for example, a portable telephone andthe like which can transmit and receive signals in a plurality of bands.

A radio communication system such as a portable telephone uses a PLL(phase locked loop) circuit which has a VCO for generating anoscillating signal at a predetermined frequency. The oscillating signalis combined with a reception signal and a transmission signal.Conventional portable telephones include a dual-band portable telephonewhich can handle signals in two frequency bands, for example, a GSM(Global System for Mobile Communication) signal in a band of 880–915 MHzand a DCS (Digital Cellular System) signal in a band of 1710–1785 MHz.Some dual-band portable telephones are designed to support two differentbands with a single PLL circuit by switching the frequency of the PLLcircuit.

In recent years, however, a need exists for a triple-band portabletelephone which can handle, for example, a PCS (Personal CommunicationSystem) signal in a band of 1850–1915 MHz in addition to the GSM and DCSsignals. It is also contemplated that the portable telephones arerequired to support a larger number of bands in the future.

For a high frequency semiconductor integrated circuit (hereinaftercalled the “high frequency IC”) designed to modulate a transmissionsignal and demodulate a reception signal, for use in such a portabletelephone which can support a plurality of bands, a direct conversionsystem is effective from a viewpoint of a reduction in the number ofparts. While the direct conversion system is relatively easy insupporting a plurality of bands, a VCO should be capable of oscillatingover a wide frequency range. In this event, when a single VCO is usedwith the intention to cover the overall frequency range, the resultingVCO would be extremely sensitive to a control voltage applied thereto,and therefore vulnerable to extraneous noise and fluctuations in a powersupply voltage.

On the other hand, a reduction in the number of parts may be effectivelyaccomplished by forming a VCO, which has been typically fabricated in amodule separate from a high frequency IC in many cases, on the samesemiconductor chip on which the high frequency IC is fabricated.However, since an on-chip VCO manufactured by the current technologiesexperiences large variations in the absolute value of the oscillatingfrequency, the on-chip VCO must be provided with a function ofcorrecting the oscillating frequency after the manufacturing. However,if the variations are corrected by trimming based on a mask option or abonding wire option, typically used in conventional semiconductorintegrated circuits, the cost is inevitably increased.

To solve these problems, the inventors have previously developed acommunication semiconductor integrated circuit (high frequency IC)comprising a PLL circuit, and filed a PCT application No.PCT/GB2002/005152 on Nov. 13, 2002 based on U.K. Patent Application No.0127537.9. The PCT application is pending but not admitted as the priorart. In this communication semiconductor integrated circuit, anoscillator circuit (RFVCO) for generating a high frequency signal foruse in transmission and reception is desired to operate in a pluralityof bands. The oscillating frequency of the oscillator circuit ismeasured in each of the bands while the oscillator circuit is appliedwith a control voltage fixed at a predetermined value, and stored in astorage circuit. A set value for specifying the frequency, appliedduring a PLL operation, is compared with the measured frequency valuesstored in the storage circuit to determine a band which is actually usedin the oscillator circuit from the result of the comparison. Theresulting communication semiconductor integrated circuit does not have ahigher sensitivity to the control voltage, and therefore is lesssusceptible to extraneous noise and fluctuations in the power supplyvoltage even with a wider frequency range available for the VCO tooscillate for supporting a plurality of communication schemes. Inaddition, the communication semiconductor integrated circuit canautomatically correct variations in the oscillating frequency of the VCOin an internal circuit.

SUMMARY OF THE INVENTION

A high frequency IC for use in a portable telephone typically has atransmission related circuit and a reception related circuit which areintegrated on a single semiconductor chip. The prior application alsodiscloses a high frequency IC which has a transmission related circuitintegrated on a single semiconductor chip together with a receptionrelated circuit. However, the high frequency IC disclosed in the priorapplication has a transmission VCO as an externally mounted circuitthough an RFVCO is integrated on the chip. The RFVCO precedes thetransmission VCO in the integration on the chip for the reasons setforth below.

First, in a high frequency IC which can support two communicationschemes, GSM and DCS, intended by the inventors for development, theRFVCO oscillates in a higher frequency band than the transmission VCO,so that larger advantages are provided by fabricating the on-chip RFVCOin view of the power consumption. Second, an on-chip VCO involves acircuit for correcting inherent variations in frequency, whereas a PLLcircuit for generating a frequency signal which is mixed with areception signal for downconverting the reception signal is oftenprovided with a frequency division counter for dividing an oscillatingsignal of the VCO to measure the frequency for locking a PLL loop at adesired frequency found from a comparison with a reference signal. Thisfrequency division counter may be used to create a circuit for measuringthe frequency to correct variations in frequency. However, due to lackof such a frequency division counter, a transmission PLL circuit must beadditionally provided with a circuit for measuring the frequency tocorrect variations in frequency. As such, a significant increase in thecircuit scale is anticipated from the addition of the circuit formeasuring the frequency.

On the other hand, since the transmission VCO is required to oscillatein a wider frequency band than the RFVCO, two transmission VCOs areneeded, one for GSM and one for DCS. Therefore, from a viewpoint of areduction in the number of parts, an on-chip transmission VCO is moreeffective than an on-chip RFVCO. Bearing this in mind, the inventorsconsidered to mount a transmission VCO on a high frequency IC suitablefor a multi-band portable telephone in order to further reduce thenumber of parts. The present invention was made in course of thisconsideration.

Technical challenges involved in mounting a transmission VCO on a highfrequency IC for a multi-band portable telephone includes:

(1) the transmission VCO should be able to oscillate over a widefrequency range;

(2) the transmission VCO should be able to correct the oscillatingfrequency for variations;

(3) the area of a chip mounted with the transmission VCO should bereduced as much as possible;

(4) a band used by the transmission VCO can be determined in a shorttime; and so forth.

The aforementioned prior application discloses a circuit for correctingthe frequency for variations associated with the fabrication of theon-chip RFVCO. While this technique may be applied to the transmissionVCO to reduce the variations in frequency, a similar circuit forcorrecting the frequency for variations, if provided for thetransmission VCO, would result in a prohibitively large circuit scale.Particularly, in a system considered by the inventors, the reception PLLcircuit comprises a counter for frequency division, but the transmissionPLL circuit does not comprise a counter for frequency division, so thatthe circuit scale would be further increased if the transmission PLLcircuit is provided with a circuit for counting the frequency forcorrection, as disclosed in the prior application.

Also, the prior application described above, since the transmission VCOis externally mounted to the high frequency IC, a band used by thetransmission VCO is directly selected by the baseband circuit. However,when the transmission VCO is built in the high frequency IC, an internalcontrol circuit must be responsible for the selection of the band to beused by the transmission VCO. In this event, an instruction may be sentfrom the baseband circuit to the high frequency IC for selecting a band,in which case a significantly long time could be taken from the time thebaseband circuit generates the instruction to the time a band to be usedby the transmission VCO is actually determined due to a time requiredfor the transmission of the instruction signal, a time required for thecontrol circuit to decode the instruction, and the like. Particularly,when an instruction to the high frequency IC is made up of serial data,the transmission of the instruction takes a relatively long time.

It is an object of the present invention to provide a communicationsemiconductor integrated circuit (high frequency IC) which is capable ofselecting a band to be used by a transmission oscillator circuit (VCO)in a short time when the transmission oscillator circuit, forming partof a transmission PLL circuit, is fabricated on a single semiconductorchip together with other oscillator circuits such as an intermediatefrequency oscillator circuit.

It is another object of the present invention to provide a communicationsemiconductor integrated circuit which is capable of reducing the burdenon a baseband circuit for determining a band to be used by atransmission oscillator circuit, when the transmission oscillatorcircuit, forming part of a transmission PLL circuit, is fabricated on asingle semiconductor chip together with other oscillator circuits suchas an intermediate frequency oscillator circuit.

It is a further object of the present invention to provide acommunication semiconductor integrated circuit which is capable ofcommunicating signals in a plurality of frequency bands, and comprises aplurality of oscillator circuits which can be formed on the samesemiconductor chip to thereby reduce the number of parts.

The above and other objects, and novel features of the invention willbecome apparent from the following description of the specification andthe accompanying drawings.

According to one aspect of the present invention, a communicationsemiconductor integrated circuit has an oscillator circuit, which formspart of a transmission PLL circuit, fabricated on a single semiconductorchip together with an oscillator circuit forming part of a reception PLLcircuit and an oscillator circuit for an intermediate frequency. Theoscillator circuit forming part of the transmission PLL circuit isconfigured to be operable in a plurality of bands. The communicationsemiconductor integrated circuit includes a circuit for measuring theoscillating frequency of the oscillator circuit which forms part of thetransmission PLL circuit, and means for storing the result of themeasurement made by the measuring circuit. A band to be used by theoscillator circuit forming part of the transmission PLL circuit isdetermined based on values for setting the oscillating frequencies ofthe oscillator circuit forming part of the reception PLL circuit and theintermediate frequency oscillator circuit, and the stored result of themeasurement.

According to the communication semiconductor integrated circuit in theforegoing aspect, since the communication semiconductor integratedcircuit internally determines a band to be used by the oscillatorcircuit forming part of the transmission PLL circuit, no instruction isrequired from an external baseband circuit or the like, therebyeliminating the time required to send the instruction. Consequently, aband to be used by the oscillator circuit can be selected in a shorttime. Also, the baseband circuit is not required to determine a band tobe used by the oscillator circuit forming part of the transmission PLLcircuit, and no instruction need be sent from the external basebandcircuit because the communication semiconductor integrated circuitinternally determines a band to be used by the transmission oscillatorcircuit, so that the communication semiconductor integrated circuit canreduce the burden on the baseband circuit for selecting the band to beused by the transmission oscillator circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an exemplary configuration of amulti-band communication semiconductor integrated circuit (highfrequency IC) according to one embodiment of the present invention, anda radio communication system using the communication semiconductorintegrated circuit;

FIG. 2 is a block diagram illustrating one embodiment of a PLL circuitincluding an RFVCO in the multi-band communication semiconductorintegrated circuit (high frequency IC) in the embodiment of FIG. 1;

FIG. 3 is a block diagram illustrating one embodiment of a PLL circuitincluding an IFVCO and a PLL circuit including a TXVCO in the multi-bandcommunication semiconductor integrated circuit (high frequency IC) inthe embodiment of FIG. 1;

FIG. 4 is a graph showing the relationship between a control voltage Vcand an oscillating frequency fRF when a variable frequency range for theRFVCO is continuously changed and when it is changed intermittently in aplurality of bands;

FIG. 5 is a graph showing the relationship between the control voltageVc and oscillating frequency fRF when the variable frequency range ofthe IFVCO is changed intermittently in a plurality of bands;

FIG. 6 is a graph showing the relationship between the control voltageVc and oscillating frequency fRF when a variable frequency range of theTXVCO is changed intermittently in a plurality of bands;

FIG. 7 is a timing chart showing timings at which the frequency of eachVCO is measured, and the frequency characteristic is corrected (a bandto be used is determined) based on the result of the measurement in aradio communication system which employs the high frequency IC accordingto one embodiment of the present invention;

FIG. 8 is a timing chart showing in greater details the timing at whichthe frequency of each VCO is measured; and

FIG. 9 is a flow chart illustrating an exemplary procedure for measuringthe frequency of the TXVCO;

FIG. 10 is a block diagram illustrating the configuration of a mainportion of a communication semiconductor integrated circuit for showingthe relationship among RFPPL, TXPPL, and IFPPL;

FIG. 11 is an explanatory diagram schematically showing how a frequencyband is determined for the TXVCO;

FIG. 12 is a circuit diagram illustrating a specific example of acircuit which implements the transmission VCO (the tuning inductorsOFF-chip); and

FIG. 13 is a circuit diagram illustrating a specific example of acircuit which implements the transmission VCO (the tuning inductorsON-chip).

DESCRIPTION OF THE EMBODIMENTS

In the following, one embodiment of the present invention will bedescribed with reference to the accompanying drawings.

FIG. 1 is a block diagram illustrating an exemplary configuration of amulti-band communication semiconductor integrated circuit (highfrequency IC) according to one embodiment of the present invention, anda radio communication system using the communication semiconductorintegrated circuit.

The radio communication system illustrated in FIG. 1 comprises anantenna 100 for transmitting and receiving signal radio waves; a switch110 for switching transmission and reception; high frequency filters 120a– 120 c such as SAW filters for removing unwanted waves from areception signal; a high frequency power amplifier 130 for amplifying atransmission signal; a high frequency IC 200 for demodulating areception signal and modulating a transmission signal; and a basebandcircuit (LSI) 300 for converting transmission data to I, Q signals andcontrolling the high frequency IC 200. The high frequency IC 200 isfabricated on a single semiconductor chip as a semiconductor integratedcircuit.

Though not particularly limited, the high frequency IC 200 in thisembodiment is designed for modulation and demodulation of signals inaccordance with four communication schemes: GSM850, GSM900, DSC1800, andPCS1900. In correspondence, the radio communication system comprises thehigh frequency filter 120 a for passing a reception signal for a GSMfrequency band; the filter 120 b for passing a reception signal in aDSC1800 frequency band; and the filter 120 c for passing a receptionsignal in a PSC1900 frequency band. For operation on 850 MHz and 900MHz, using the LNA 210 a, the designer would have an 850 MHz filter anda 900 MHz filter and select using a switch.

The high frequency IC 200 in this embodiment is roughly composed of areception related circuit RXC; a transmission related circuit TXC; and acontrol related circuit CTC which includes other circuits common to thetransmission and reception such as a control circuit, a clock relatedcircuit, and the like.

The reception related circuit RXC comprises low noise amplifiers 210 a,210 b, 210 c each for amplifying a reception signal; a phase dividercircuit 211 for dividing an oscillating signal φRF generated by a highfrequency oscillator circuit (RFVCO) 250 to generate orthogonal signalswhich are 90° out-of-phase from each other; demodulator circuits 212 a,212 b each including a mixer for combining the reception signalamplified by the low noise amplifier 210 a, 210 b, 210 c with theorthogonal signals generated by the phase divider circuit 211 fordemodulation; high gain amplification units 220A, 220B for amplifyingthe demodulated I, Q signals, respectively, for delivery to the basebandcircuit 300; and an offset cancel circuit 213 for canceling input DCoffsets of the amplifiers within the high gain amplification units 220A,220B.

The high gain amplification unit 220A comprises a plurality of low passfilters LPF 11, LPF 12, LPF 13, LPF 14 and gain control amplifiers PGA11, PGA 12, PGA 13, which are alternately connected in series; and anamplifier AMP1 with a fixed gain connected at the final stage. The highgain amplification unit 220A amplifies the I signal and outputs theamplified I signal to the baseband circuit 300. Likewise, the high gainamplification unit 220B comprises a plurality of low pass filters LPF21, LPF 22, LPF 23, LPF 24 and gain control amplifiers PGA 21, PGA 22,PGA 23, which are alternately connected in series; and an amplifier AMP2with a fixed gain connected at the final stage, and amplifies the Qsignal and outputs the amplified Q signal to the baseband circuit 300.

The offset cancel circuit 213 comprises A/D converter circuits (ADC)provided in correspondence to the gain control amplifiers PGA 11–PGA 23,respectively, for converting output potential differences, when theirinput terminals are short-circuited, to digital signals; DA convertercircuits (DAC) each for generating an input offset voltage to reduce DCoffsets in the outputs of the corresponding gain control amplifiers PGA11–PGA 23 to zero based on the results of conversions made by the ADconverters, and applying the input offset voltages to differentialinputs; and a control circuit for controlling the AD converter circuits(ADC) and DA converter circuits (DAC) to perform an offset cancelingoperation.

The transmission related circuit TXC comprises an oscillator circuit(IFVCO) 230 for generating an oscillating signal φIF at an intermediatefrequency, for example, 640 MHz; a frequency divider circuit 231 fordividing the oscillating signal φIF generated by the oscillator circuit230 by a factor of four to generate a signal at 160 MHz; a phase dividercircuit 232 for further dividing the signal divided by the frequencydivider circuit 231 to generate orthogonal signals which are 90°out-of-phase from each other; modulator circuits 233 a, 233 b formodulating the generated orthogonal signals with the I signal and Qsignal supplied from the baseband circuit 300; an adder 234 forcombining the modulated signals; a transmission oscillator circuit(TXVCO) 240 for generating a transmission signal φTX at a predeterminedfrequency; an offset mixer 236 for combining a feedback signal extractedby a coupler or the like from the transmission signal φTX outputted fromthe transmission oscillator circuit (TXVCO) 240 with a signal φRF′generated by dividing the oscillating signal φRF generated by the highfrequency oscillator circuit (RFVCO) 250 to generate a signal at afrequency which is equal to the difference in frequency between thefeedback signal and signal φRF′; an analog phase comparator 237 a and adigital phase comparator 237 b for comparing the output of the offsetmixer 236 with a signal TXIF generated by the adder 234 from acombination of the modulated signals to detect a phase difference; and aloop filter 238 for generating a voltage in accordance with the outputsof the phase detector circuits 237 a, 237 b.

The loop filter 238 includes a resistor and a capacitor which areconnected to associated external terminals of the high frequency IC 200of this embodiment as external elements. The transmission oscillatorcircuit (TXVCO) 240 comprises an oscillator circuit 240 a for generatingtransmission signals for GSM850 and GSM900; and an oscillator circuit240 b for generating transmission signals for DCS1800 and PCS1900. Thatis, the oscillator circuit 240 a is used for a first frequency band (theGSM 850 and the GSM 900), and the oscillator 240 b is used for a secondfrequency band (the DCS1800 and the PCS1900) which is different from thefirst frequency band in a frequency.

The analog phase comparator 237 a and digital phase comparator 237 b areprovided for promoting a draw-in operation at the time the PLL circuitstarts the operation. Specifically, the digital phase comparator 237 bis first used for phase comparison upon start of transmission, and issubsequently switched to the analog phase comparator 237 a such that thephase loop can be rapidly locked.

The chip on which the high frequency IC 200 of this embodiment isfabricated further comprises a control circuit 260 for controlling theentire chip; an RF synthesizer 261 which constitutes an RF PLL circuittogether with the high frequency oscillator circuit (RFVCO) 250; an IFsynthesizer 262 which constitutes an IF PLL circuit together with theintermediate frequency oscillator circuit (IFVCO) 230; and a referenceoscillator circuit (VCXO) 264 for generating a clock signal φref whichserves as a reference signal for these synthesizers 261, 262. Thesynthesizers 261, 262 are each composed of a phase comparator circuit, acharge pump, a loop filter, and the like.

Since the reference oscillating signal φref is required to be highlyaccurate in frequency, an external quartz oscillator is connected to thereference oscillator circuit 264. A frequency such as 26 MHz or 13 MHzmay be selected for the reference oscillating signal φref.

In FIG. 1, blocks labeled fractions such as ½, ¼ and the like representfrequency divider circuits, respectively, while a block labeled BFFrepresents a buffer circuit. Blocks labeled SW1, SW2, SW3 representswitches which are switched for a GSM mode for transmitting andreceiving signals in accordance with the GSM scheme, and a DCS/PCS modefor transmitting and receiving signals in accordance with the DCS or PCSscheme to select a frequency division ratio for a signal to becommunicated. A block labeled SW4 represents a switch which iscontrolled ON/OFF to supply the I, Q signals from the baseband circuit300 to the modulation mixers 233 a, 233 b upon transmission. Theseswitches SW1–SW4 are controlled by signals from the control circuit 260.

The control circuit 260 is provided with a control register CRG which isset based on a signal from the baseband circuit 300. Specifically, thecontrol circuit 260 is supplied from the baseband circuit 300 with aclock signal CLK for synchronization, a data signal SDATA, and a loadenable signal LEN as a control signal for the high frequency IC 200. Asthe load enable signal LEN is asserted to an effective level, thecontrol circuit 260 sequentially fetches the data signal SDATAtransmitted thereto from the baseband circuit 300 in synchronism withthe clock signal CLK, and sets the data signal SDATA in the controlregister CRG. Though not particularly limited, the data signal SDATA maybe serially transmitted. The baseband circuit 300 is mainly composed ofa microprocessor.

Though not particularly limited, the control register CRG may beprovided with a control bit for controlling the high frequencyoscillator circuit (RFVCO) 250 and intermediate frequency oscillatorcircuit (IFVCO) 230 to start a measurement of the frequency of the VCO;a bit field for specifying a mode such as a reception mode, atransmission mode, an idle mode, a warm-up mode, and the like. Here, theidle mode is set to enter a sleep state in which only an extremely smallnumber of circuits are left operative while a majority of circuitsincluding at least the oscillator circuits are inoperative, such as in awaiting time. The warm-up mode is set to start the PLL circuitsimmediately before transmission or reception.

In this embodiment, a transmission PLL circuit (TXPLL) for convertingthe frequency is composed of the phase detector circuits 237 a, 237 b;loop filter 238; transmission oscillator circuits (TXVCO) 240 a, 240 b;and offset mixer 236. In the multi-band radio communication system inthis embodiment, for example, in response to a command from the basebandcircuit 300, the control circuit 260 changes the frequency φRF of theoscillating signal from the high frequency oscillator circuit 250 inaccordance with a channel to be used upon transmission/reception, andswitches the switch SW2 in accordance with the GSM mode or DCS/PCS modeto change the frequency of the signal supplied to the offset mixer 236,thereby switching the transmission frequency.

Table 1 shows exemplary frequencies set for the oscillating signals φIF,φTX, φRF generated by the intermediate frequency oscillator circuit(IFVCO) 230, transmission oscillator circuit (TXVCO) 240, and highfrequency oscillator circuit (RFVCO) 250, respectively, in the quad-bandhigh frequency IC of this embodiment.

TABLE 1 RFVCO (MHz) IFVCO TXIF TXVCO TRANS- (MHz) (MHz) (MHz) RECEPTIONMISSION GSM850 640 80 824 3476 3616 640 80 849 3576 3716 GSM900 640 80880 3700 3840 640 80 915 3840 3980 DCS1800 640 80 1710 3610 3580 640 801785 3760 3730 PCS1900 640 80 1850 3860 3860 640 80 1910 3980 3980

As shown in Table 1, the oscillating frequency of the intermediatefrequency oscillator circuit (IFVCO) 230 is set at 640 MHz for any ofGSM, DCS, PCS in this embodiment. Also, the IC can use 648 MHz or 656MHz. The oscillating signal at 640 MHz is divided by the frequencydivider circuit 231 and phase divider circuit 232 by a factor of eight,respectively, to generate a carrier (TXIF) at 80 (81, 82) MHz formodulation.

On the other hand, the oscillating frequency of the high frequencyoscillator circuit (RFVCO) 250 is set at different values for areception mode and a transmission mode, respectively. In thetransmission mode, the oscillating frequency fRF of the high frequencyoscillator circuit (RFVCO) 250 is set, for example, in a range of 3616to 3716 MHz for GSM850; in a range of 3840 to 3980 MHz for GSM900; in arange of 3610 to 3730 MHz for DCS; and in a range of 3860 to 3980 MHzfor PCS. Then, the oscillating frequency fRF is divided by the frequencydivider circuit by a factor of four for GSM; and by a factor of two forDCS and PCS. The resulting signal is supplied to the offset mixer 236 asφRF′.

FIG. 10 illustrates the RFPLL, TXPLL, and IFPLL extracted from thecircuit of FIG. 1. As can be seen from FIG. 10, the offset mixer 236outputs a feedback signal (fRF′-fTX) corresponding to the difference infrequency between the φRF′ and the transmission oscillating signal φTXfrom the transmission oscillator circuit 130, and the transmission PLL(TXPLL) operates to match the frequency of the feedback signal with thefrequency of the modulated signal TXIF. In other words, the TXVCO 240 iscontrolled to oscillate at a frequency corresponding to the differencebetween the frequency (fRF/4 for GSM, fRF/2 for DCS and PCS) of theoscillating signal φRF′ from the RFVCO 250 and the frequency (fIF′) ofthe modulated signal TXIF. This is the transmission operation in asystem generally referred to as the offset PLL system.

FIG. 2 illustrates a specific example of a PLL circuit which has afunction of measuring the frequency of the VCO, and a function ofcorrecting the VCO for the frequency characteristic based on the resultof the measurement. The PLL circuit illustrated in FIG. 2 comprises ahigh frequency oscillator circuit (RFVCO) 250; a variable frequencydivider circuit 12 for dividing the oscillating signal φRF from theRFVCO 250; a fixed frequency divider circuit 13 for dividing thereference oscillating signal φref from the reference oscillator circuit264 by a factor of 65; a phase comparator 14 for comparing in phase thesignal divided by the variable frequency divider circuit 12 with thesignal divided by the fixed frequency divider circuit 13 to output avoltage UP, DOWN in accordance with the difference in phase between thetwo signals; a charge pump 15; and a loop filter 16. The charge pump 15charges up a capacitive element of the loop filter 16 to output acontrol voltage Vc for the RFVCO 250 which responsively oscillates at apredetermined frequency. In this manner, a PLL loop is formed. Thecapacitive element or capacitor and a resistor, which constitute theloop filter 16, are connected as external elements.

As illustrated in FIG. 2, the PLL circuit in this embodiment comprises aswitch SW0 placed between the charge pump 15 and loop filter 16 forsupplying the loop filter 16 with a predetermined DC voltage VDC, inplace of the voltage Vc from the charge pump 15, upon measurement of thefrequency and PLL draw-in operation; and a DC voltage source 17 forgenerating a DC voltage VDC which is applied to the charge pump 15. ThePLL circuit further comprises a storage circuit 18 comprised ofregisters or the like for storing values counted by the variablefrequency divider circuit 12; a suitable band decision circuit 19 forcomparing the frequency values stored in the storage circuit 18 with setvalues N8–N0 and A5, A4 set in the counter 22 from the outside togenerate a band switching signal VB3–VB0 for the RFVCO 250; and thelike. The suitable band decision circuit 19 may be part of the controlcircuit 260.

For measuring the frequency, the DC voltage VDC supplied to the loopfilter 16 through the switch SW0 may have any voltage value as long asit is within an effective variable range of the control voltage Vc. Inthis embodiment, an upper limit value (Vcp-max) in the variable range ofthe control voltage Vc is selected for the DC voltage VDC. During ameasurement of the frequency, the DC voltage VDC is maintained at thesame value even when a band is switched to another. The switch SW0,variable frequency divider circuit 12, storage circuit 18, and suitableband decision circuit 19 are controlled by the control circuit 260. Thevariable frequency divider circuit 12, fixed frequency divider circuit13, phase comparator 14, charge pump 15, storage circuit 18, andsuitable band decision circuit 19 make up the RF synthesizer 261illustrated in FIG. 1.

The RFVCO 250 comprises, for example, a Colpitts oscillator circuitusing an LC resonance circuit. A plurality of capacitive elements, eachforming part of the LC resonance circuit, are arranged in parallelthrough respective switching elements associated therewith. Theswitching elements may be selectively turned on with the band switchingsignal VB3–VB0 to switch a connected capacitive element, i.e., the valueC of the LC resonance circuit, thereby switching the oscillatingfrequency step by step. On the other hand, the RFVCO 250 has a variablecapacitance diode as a variable capacitance element, the capacitance ofwhich is changed by the control voltage Vc from the loop filter 16 tocontinuously change the oscillating frequency.

When a frequency range covered by the VCO is extended only with a changein the capacitance of the variable capacitance diode through the controlvoltage Vc, a resulting Vc-fRF characteristic exhibits an abrupt slope,as indicated by a broken line A in FIG. 4, to cause an increase in thesensitivity of the VCO, i.e., the ratio of a frequency changing amountto a control voltage changing amount (Δf/ΔVc), so that the VCO becomesmore vulnerable to noise. In other words, slight noise introduced intothe control voltage Vc would result in a large change in the oscillatingfrequency fRF of the VCO.

To solve this problem, the RFVCO 250 in this embodiment comprises aplurality of capacitive elements, which form part of the LC resonancecircuit, in parallel to switch a used capacitive element in n stageswith the band switching signal VB3–VB0 to change the value C, to controlthe oscillation along a plurality of Vc-fRF characteristic curves asindicated by solid lines in FIG. 4. Moreover, in this embodiment, thestorage circuit 18 and suitable band decision circuit 19 provided in theRFVCO 250 eliminate a adjustment operation called a frequency alignmentwhich has been required in conventional PLL circuits.

Specifically, a conventional PLL circuit operates the VCO to measure thefrequency and align the frequency such that each of a plurality ofVc-fRF characteristic curves has a predetermined initial value and apredetermined slope even when a VCO is tuned to have a plurality ofVc-fRF characteristic curves, for example, as shown in FIG. 4. On thecontrary, the PLL circuit in this embodiment previously switches theswitch SW0 to apply the RFVCO 250 with the predetermined DC voltage VDC,and measures the frequency in each band for storage in the storagecircuit 18. For an actual use, the PLL circuit compares the set valuesN8–N0 and A5, A4 applied to the counter 22 from the outside inaccordance with a specified band with the measured frequencies stored inthe storage circuit 18 to select only one which can cover the frequencyrange of the specified band from the plurality (n) of Vc-fRFcharacteristic curves, as shown in FIG. 4, to switch the RFVCO(capacitive element) to oscillate in accordance with the selectedcharacteristic curve.

According to this strategy, the RFVCO may be designed with n Vc-fRFcharacteristic curves, each of which covers a frequency range slightlywider than an intended frequency range in consideration of variations,and which slightly overlap (preferably, half by half) the frequencyranges with adjacent ones, as shown in FIG. 4, to provide without fail acharacteristic curve which can cover a specified frequency range.Therefore, a Vc-fRF characteristic curve corresponding to a particularspecified band may be selected based on the actual characteristic foundby a measurement, thereby eliminating the alignment of the frequency anda previous one-to-one correspondence of a used band to a switching stateof the RFVCO.

The variable frequency divider circuit 12 comprises a prescaler 21 fordividing the oscillating signal of the RFVCO 250; and a modulo counter22 comprised of a first counter 22N and a second counter 22A for furtherdividing the signal divided by the prescaler 21.

The division of the oscillating signal by the prescaler 21 and modulocounter 22 is a known technique. The prescaler 21 can divide theoscillating signal at two different frequency division ratios, forexample, 1/64 and 1/65. One frequency division ratio is switched to theother by a count end signal of the second counter 22A. The first counter22N and second counter 22A are programmable counters. The first counter22N is loaded with the quotient (integer part) resulting from a divisionof a desired frequency (the oscillating frequency fRF of the VCO desiredas the output) by the frequency fref′ of the reference oscillatingsignal φref′ and the first frequency division ratio (64 in theembodiment) of the prescaler 21, while the second counter 22A is loadedwith the residue (MOD) resulting from the division. Each of the countersterminates the counting as it has counted the value set therein, andagain begins counting up to the set value.

Specifically, assuming, for example, that the reference oscillatingsignal φref′ has the frequency fref′ at 400 kHz, and a desiredoscillating frequency fRF of the VCO is at 3789.6 MHz, the first counter22N is loaded with the value N equal to “148” while the second counter22A is loaded with the value A equal to “2” since 3789.6/(0.4*64)=148and a residue of 2. As the prescaler 21 and modulo counter 22 operatewith such values set in the respective counters, the prescaler 21 firstdivides the oscillating signal of the RFVCO 250 by a factor of 64, andthe second counter 22A counts the output up to “2” which is set therein,at which time the second counter 22A outputs a count end signal MC whichcauses the prescaler 21 to switch the operation. Consequently, theprescaler 21 divides the oscillating signal of the RFVCO 250 by a factorof 65 until the second counter 22A again counts the output of theprescaler 21 up to the set value “2.”

The foregoing operation permits the modulo counter 22 to divide theoscillating signal not only in an integer ratio but also in a fractionalratio. The PLL circuit in this embodiment applies a feedback to matchthe frequency of the output of the first counter 22N with the frequencyfref′ (400 kHz) of the reference oscillating signal φref′ to control theoscillation of the RFVCO 250. Therefore, in the foregoing specificexample in which the first counter 22N is loaded with the value N equalto “148” and the second counter 22A with the value A equal to “2,” theoscillating frequency fRF of the RFVCO 250 is calculated to be 3789.6MHz as follows:fRF=(64×148+2)×fref′=9474×400=3789600

It should be noted that the first counter 22N and second counter 22A areactually comprised of binary counters, so that the value N set in thefirst counter 22N and the value A set in the second counter 22A areapplied in binary codes. Though not particularly limited, in thisembodiment, the first counter 22N operates as a 9-bit counter, while thesecond counter 22A as a 6-bit counter during a PLL operation, so thatthe value set in the first counter 22N is given by a 9-bit code N8–N0,while the value set in the second counter 22A is given by a 6-bit codeA5–A0.

Further, in this embodiment, the first counter 22N can operate also asan 11-bit counter for measuring the frequency. The RFVCO 250 can switchthe oscillating frequency in 16 bands, i.e., in 16 stages, so that thestorage circuit 18 comprises 15 registers REG0–REG14 for storingfrequencies measured for the 16 bands, respectively. So, it is notnecessary to measure every RFVCO band. If the RFVCO has 16 switchedbands, it is only necessary to measure calibration values for 15 bands(for example bands #0 up to #14, not measuring band #15). For the RFVCO,we can measure bands #0, #1, #2, #3 up to band #14. Alternatively, wecan use a ‘band-skipping’ system, measuring band #0, #2, #4, #6, . . .up to #14. The suitable band decision circuit 19 in turn comprises an11-bit comparator for comparing a value stored in the registerREG0–REG14 of the storage circuit 18 with the 9-bit code N8–N0 set inthe first counter 22N and the upper two bits A5, A4 of the 6-bit codeA5–A0 set in the second counter 22A to output a 4-bit code VB3–VB0 as aband switching signal for the RFVCO 250.

Upon measurement of the frequency, the control circuit 260 generates theswitching signal VB3–VB0 to select 16 bands in order and outputs theswitching signal VB3–VB0 to the RFVCO 250. Further, upon measurement ofthe frequency, the control circuit 260 controls the first counter 22N tooperate as an 11-bit counter, and to count the number of clocks in along term, for example, four periods of the reference oscillating signalφref′, rather than one period. Also, upon measurement of the frequency,the control circuit 260 controls the second counter 22A to stop theoperation such that the frequency division ratio of the prescaler 22 isnot switched. In this way, the prescaler 22 divides the oscillatingsignal of the VCO only by a factor of 64 for measuring the frequency.

In this embodiment, the control circuit 260 operates the first counter22N for four periods of the reference oscillating signal φref′ ratherthan one period during a measurement of the frequency in order toincrease the accuracy of measurement. Specifically, if a maximum errorpossibly occurring in the counter 22N in a measurement for one period ofthe reference oscillating signal φref′, i.e., one pulse count erroroccurs in the counter 22N in a measurement for one period of φref′, theerror at this time is enlarged by a factor of 64 which is the frequencydivision ratio of the prescaler 21 due to the provision of the prescaler21. For this reason, the maximum error of the counter 22N amounts to25.6 MHz (400 kHz×64) when the reference oscillating signal φref′ is at400 kHz. However, the error occurring in the counter 22N in ameasurement for four periods is reduced to approximately 6.4 MHz whichis one quarter as much.

The 11-bit counted value provided by the first counter 22N uponmeasurement of the frequency is stored in one of the registers in thestorage circuit 18. The stored value, the upper nine bits of which areregarded as the quotient (integer part), is compared with the code N8–N0set in the first counter 22N supplied from the outside by the suitableband decision circuit 19 during a PLL operation. Also, the lower twobits of the value stored in the register of the storage circuit 18 areregarded as a residue (fraction part) and are compared with the uppertwo bits A5, A4 of the code A5–A0 set in the second counter 22A suppliedfrom the outside by the suitable band decision circuit 19.

The suitable band decision circuit 19, which is composed of acomparator, an exclusive OR gate, and the like, decides a band to beused by the RFVCO 250 from the result of comparison of the values storedin the respective registers REG0–REG15 of the storage circuit 18 withthe set code N8–N0 and the upper two bits A5, A4 of the set code A5–A0,generates the band switching code VB3–VB0 for selecting the decidedband, and supplies the band switching code VB3–VB0 to the RFVCO 250.When the RFVCO 250 is used in a PLL circuit for use in a communicationsystem such as GSM, the respective bands are set at intervals of 400kHz, for example, in compliance with the channel intervals of GSM.

The following description will focus on a procedure for measuring thefrequency and a correcting the frequency characteristic, under thecontrol of the control circuit 260, in the PLL circuit of thisembodiment. The measurement of the frequency for the RFVCO 250, and thecorrection for the frequency characteristic based on the result of themeasurement are made, for example, each time a predetermined command isinputted from the baseband circuit 300 in an idle mode.

Upon start of a measurement of the frequency for the RFVCO 250, thecontrol circuit 260 first switches the switch SW0 to supply the loopfilter 16 with the DC voltage VDC. Then, the control circuit 260 waitsfor the voltage Vc of the loop filter 16 to stabilize as well as for theoscillating frequency of the RFVCO 250 to stabilize. Next, the controlcircuit 260 fixes the frequency division ratio of the prescaler 21 to1/64, and sets the first counter 22N to operate as an 11-bit counter.Next, the control circuit 260 references a pointer indicative of aselected band to output the code VB3–VB0 for selecting a band of theRFVCO 250. Here, the band selected first is, for example, BAND0 whichaccounts for the lowest frequency range.

Next, the control circuit 260 forces the first counter 22N to count forfour periods of the reference oscillating signal φref′, and stores thecounted value of the first counter 22N in one of the registers of thestorage circuit 18. The register first loaded with the counted value isthe first register REG0. Then, the control circuit 260 determineswhether or not the frequencies have been measured for all the bands. Ifnot, the control circuit 260 increments the pointer indicative of theselected band by two (+2) and repeats the foregoing operation.

Subsequently, as the PLL circuit is supplied with a frequency settingvalue in accordance with a used channel from the baseband circuit uponstart of transmission/reception in a standby state, the suitable banddecision circuit 19 decides a band to be used by the RFVCO 250 based onthe frequency setting value from the result of comparison of the storedvalues in the respective registers REG0–REG15 of the storage circuit 18with the set code N8–N0 and A5, A4. Then, the suitable band decisioncircuit 19 supplies the RFVCO 250 with the band selection signal VB3–VB0for correcting the frequency characteristic.

The high frequency IC 200 in the embodiment illustrated in FIG. 1 alsohas a function of measuring the frequencies of the intermediatefrequency VCO (IFVCO) 230 and transmission VCO (TXVCO) 240, and afunction of correcting the frequency characteristics of these VCOs 230,240 based on the result of the measurement. In addition, these functionscan be executed by a common circuit to limit an increase in the circuitscale.

The following description will focus on an embodiment of the PLL circuitwhich implements the function of measuring the frequencies of the IFVCO230 and TXVCO 240, and the function of correcting the frequencycharacteristics of the IFVCO 230 and TXVCO 240 based on the result ofthe measurement with reference to FIG. 3. In FIG. 3, circuits identicalto those shown in FIGS. 1 and 2 are designated the same referencenumerals, and repetitive description will be omitted.

As illustrated in FIG. 3, an IF synthesizer 262 is similar inconfiguration to the RF synthesizer 261 shown in FIG. 2. Specifically,the IF synthesizer 262 comprises a prescaler 31 which can divide theoscillating frequency by a factor of 16 or 17; an N counter 32N and an Acounter 32A which make up a modulo counter; a fixed frequency dividercircuit 33; and the like. In FIG. 3, circuits corresponding to the phasecomparator 14, charge pump 15, switch SW0, and loop filter 16 shown inFIG. 2 are represented as an IF PLL circuit 30. The fixed frequencydivider circuit 33 is designed to generate an operating clock (1 MHz)for the control circuit 260, other than the reference oscillating signalφref′ at 400 KHz. The operation of the N counter 32N and A counter 32Ais similar to the embodiment illustrated in FIG. 2, so that descriptionthereon is omitted.

The IF synthesizer 262 also comprises a selector 34 for selectivelysupplying the prescaler 31 with a signal generated by dividing theintermediate frequency signal φIF or a signal generated by dividing theoscillating signal φTX from the TXVCO 240 in accordance with a signalfrom the control circuit 260; a comparator circuit 35 for comparing thevalue counted by the N counter 32N with reference data (IF frequencyinformation) stored in a ROM 40 upon measurement of the frequency of theIFVCO; a counter/register 36 for holding information on a band to beused by the IFVCO 230 based on the result of the comparison in thecomparator circuit 35; a register 37 for storing the value counted bythe N counter 32N upon measurement of the frequency of the TXVCO; aprocessing circuit 38 for calculating a target oscillating frequencyvalue TX(N, A) for the TXVCO based on frequency setting values RF/IF(N,A) for the RFVCO and IFVCO supplied from the baseband circuit; and aband decision circuit 39 for comparing the value calculated by theprocessing circuit 38 with the value stored in the register 37 togenerate a code VB2–VB0 for specifying a band used in the TXVCO 240.

In FIG. 3, a switch SW0′ can supply the loop filter 238 with apredetermined DC voltage VCD in place of the voltage Vc from the chargepump upon measurement of the frequency of the TXVCO 240 or in a PLLdraw-in operation. A DC voltage source 217 generates the DC voltage VDCapplied to the charge pump 328. No circuit corresponding to the chargepump 15 shown in FIG. 2 is included in FIG. 3 because an output stage ofa phase comparator circuit 237 has a function similar to the chargepump. Either a TXVCO 240 a for GSM or a TXVCO 240 b for DCS/PCS is madeoperative by a control signal from the control circuit 260 uponmeasurement of the frequency and upon transmission.

As illustrated in FIG. 5, the target value of four bands can be in aregister inside the radio chip, programmed from baseband.

The TXVCO 240 is configured to operate in accordance with thecharacteristic in one of eight bands, as illustrated in FIG. 5. Theprocessing circuit 38 is provided for determining a band to be used bythe TXVCO from the frequency setting values RF/IF(N, A) for the RFVCOand IFVCO supplied from the baseband circuit 300, so that the basebandcircuit 300 need not supply the frequency setting value for the TXVCO,and a band to be used can be decided for the TXVCO in a short time. Theband decision circuit 39 determines a band to be used from the measuredfrequencies of the TXVCO stored in the register 37, and the targetoscillating frequency TX(N, A) for the TXVCO calculated by theprocessing circuit 38.

Now, description will be made on a specific technique for determining aband to be used by the TXVCO 240 from the frequency setting valuesRF/IF(N,A) for the RFVCO 250 and IFVCO 230, supplied from the basebandcircuit 300.

As previously described in connection with FIG. 10, the frequency fTX ofthe TXVCO 240 is expressed by the difference between the frequency fRF′of the signal φRF′ generated by dividing the oscillating signal φRF ofthe RFVCO 250 and the frequency fIF′ of the signal φIF′ generated bydividing the oscillating signal φIF of the IFVCO 230, i.e.,fTX=fRF′-fIF′. Assuming herein that frequency setting values in thecounter 22 of the RFPLL are NRF, ARF; frequency setting values in thecounter 32 of the IFPLL are NIF, AIF; the division ratios of thefrequency division circuit 21 of the RFPLL and the frequency divisioncircuit 31 of the IFPLL are DIVrf, DIVif, respectively, and thefrequencies of the reference signal φref′ supplied to the phasecomparator circuits 14, 30 of the RFPLL and IFPLL are frfre′, fifref′,the oscillating frequencies fRF, fIF of the RFVCO 250 and IFVCO 230 arerespectively expressed by:fRF=(Nrf×DIVrf+Arf)×frfref′fIF=2×(Nif×DIVif+Aif)×fifref′

The frequencies fRF′, fIF′ are respectively expressed by:fRF′=fRF/4 (GSM)fRF′=fRF/2 (DCS, PCS)fIf′=fIF/8From the above equations and the aforementioned equation fTX=fRF′-fIF′,for GSM, fTX can be calculated as follows:fTX = {(Nrf × DIVrf + Arf) × frfref^(′) × 1/4} − {2 × (Nif × DIVif + Aif) × fifref^(′) × 1/8} = Nrf × DIVrf × frfref^(′) × 1/4 − Nif × DIVif × fifref^(′) × 1/4 + (Arf × frfref^(′) × 1/4 − Aif × fifref^(′) × 1/4)Here, for example, when frfref′ and fifref′ are both at 1 MHz; thedivision ratios DIVrf, DIVif of the frequency division circuits 21, 31are “64” and “16”; and GSM900 is selected so that NRF=56, ARF=16 areset, for example, for the frequency setting values in the counter 22 ofthe RFPLL, and NIF=20, AIF=0 are set for the frequency setting values inthe counter 32 of the IFPLL, fTX is calculated from the foregoingequation as follows:fTX = (56 × 64 × 1 × 1/4) − (20 × 16 × 1 × 1/4) + (16 × 1 × 1/4 − 0 × 1 × 1/4) = 896 − 80 + 4 = 820  (HMz)

It should be noted that the above description shows an example ofcalculation using the above algorithm and, in a real system using theabove algorithm, frfref′ and fifref′ may be derived from the referencefrequency for the RF/IF PLL in use.

On the other hand, when frequency values 910, 890, 870, 80, 830, 810,790, resulting from the aforementioned measurement, have been stored inthe register 37 as measured values as shown in FIG. 11, the comparatorin the suitable band decision circuit 39 compares the value (820)calculated by the processing circuit 38 with each of the measured valuesin the register 37 in order. Then, the results of the comparisons (Highor Low) are X-ORed with one another, and a frequency band correspondingto the stored value having the X-OR result “High” is selected as a bandto be used by the TXVCO 250.

FIGS. 12 and 13 illustrate one embodiment of the LC resonance oscillatorcircuit which is used as the TXVCO 240 a, 240 b forming part of thetransmission related circuit TXC. As illustrated in FIGS. 12 and 13, theoscillator circuit in this embodiment comprises a pair of P-channel MOStransistors Q11, Q12 which have commonly connected sources, and gatesand drains cross-coupled to each other; a regulated current source Icconnected between the common source of the transistors Q1, Q2 and apower supply voltage terminal Vcc; a switch SW10 connected in serieswith the regulated current source Ic; inductors (coils) L1, L2 connectedbetween the drains of the respective transistors Q1, Q2 and a groundpoint GND, respectively; a first series circuit including a capacitorC11 connected in series between the drain terminals of the transistorsQ11, Q12, switches SW21, SW22, and a capacitor C22; a second seriescircuit connected in parallel with the first series circuit andincluding a capacitor C21, switches SW21, SW22, and a capacitor 22; athird series circuit connected in parallel with the first series circuitand including a capacitor C31, switches SW31, SW32, and a capacitor C32;and varactor diodes Dv1, Dv2 as variable capacitive elements connectedin series between the drain terminals connected in series between thedrain terminals of the transistors Q11, Q12.

The switches SW11–SW32 are controlled to turn ON/OFF by the bandselection signal VB2–VB0 from the suitable band decision circuit 39 tochange the oscillating frequency step by step. Each of the varactordiodes Dv11, Dv12 is applied with the control voltage Vc at one terminalthereof from the loop filter 238 shown in FIG. 3 to continuously controlthe frequency.

Specifically, as a larger number of the switches SW11–SW32 are turnedon, the capacitance connected between the drains of the transistors Q11,Q12 becomes larger to reduce the oscillating frequency. On the otherhand, as a smaller number of the switches SW11–SW32 are turned on, theoscillating frequency becomes higher. As the switch SW10 connected inseries with the regulated current source Ic is turned on, the oscillatorcircuit begins an oscillating operation, and as the switch SW10 isturned off, the oscillator circuit stops the oscillating operation.Instead of providing the switch SW10, the regulated current source Icmay be directly controlled ON/OFF. The switch SW10 is controlled by aswitching signal TRANSMIT_VCO_ENABLE outputted from the control circuit260.

In this embodiment, LC resonance oscillator circuits, which form part ofthe reception VCO 250 and intermediate frequency VCO 230, use inductorsmounted on the chip. As for the transmission VCOs 240 a, 240 b, theinductors L1, L2 of the transmission VCO 240 a are externally mounted,and the inductors L1, L2 of the transmission VCO 240 b are built in theIC. The varactor diodes Dv11, Dv12 may be implemented by P-channel MOStransistors.

As shown in Table 1, the frequency of the IFVCO may be essentiallyconstant. However, in an actual system, harmonics of the signalgenerated by the reference oscillator circuit 264, and beat noisecorresponding to the difference in frequency between the harmonics andthe intermediate frequency signal can superimpose on the oscillatingsignal generated by the IFVCO 230, or introduce into the modulatorcircuit 233 to degrade the CN ratio, depending on the frequency of aused channel. Even in such a situation, the frequency of the IFVCO maybe changed, for example, from 640 MHz to 648 MHz or 656 MHz to reducethe noise and improve the CN ratio. It is therefore quite effective toconfigure the IFVCO 230 to operate in a plurality of bands as in thisembodiment, so that a band to be used can be selected from the availablebands.

Next, referring to FIG. 7, description will be made on the timings atwhich the frequency is measured for each VCO, and the frequencycharacteristic is corrected (a band to be used is decided) based on theresult of the measurement in the radio communication system whichemploys the high frequency IC in this embodiment.

In FIG. 7, “Idle” indicates an idle mode in which the radiocommunication system does not transmit or receive as in a waiting time;“Warm up” indicates a warm-up mode in which the PLL is started andlocked before transmission or reception; “Rx” indicates a reception modein which circuits associated with reception are operated to receive asignal; and “Tx” indicates a transmission mode in which circuitsassociated with transmission are operated to transmit a signal. Thesemodes are started in response to a command supplied from the basebandcircuit 300 to the control circuit 260 of the high frequency IC 200. Thecommand may be comprised of a code of a predetermined bit length such aseight bits (hereinafter called the “Word”), and a plurality of commandcodes have been previously provided for the command.

After powering on, as the baseband circuit 300 supplies the highfrequency IC 200 with a command “System Reset Word”. The control circuit260 resets circuits such as the registers within the high frequency IC200, so that the high frequency IC 200 enters the idle mode (at timingt1 in FIG. 7). This idle mode is a low power consumption mode in whichthe respective VCOs are prohibited from oscillating.

Next, as the control circuit 260 is supplied with a command “VCOCalibration Word” comprised of a predetermined bit code, a measurementof the frequency is started for each band assigned to the RFVCO 250 andIFVCO 230 (at timing t2). In the high frequency IC 200 of thisembodiment, the RFVCO 250 is assigned 16 bands while the IFVCO 230 isassigned eight bands, so that the measurement of the frequencies of theIFVCO 230 finishes earlier than that of the RFVCO 250 (at timing t3).Therefore, a measurement of the frequency of the transmission TXVCO 240a is automatically started using the counters 32N, 32A which have beenused for measuring the frequency of the IFVCO 230. Upon terminating themeasurement of the frequency of the TXVCO 240 a, a measurement of thefrequency is started for the TXVCO 240 b (at timing t4). A band to beused is selected for the IFVCO 230 immediately after the frequencieshave been measured for the IFVCO 230.

After transmission of “VCO Calibration Word”, the baseband circuit 300sends “System Configuration Word” for instructing initial settings aftera proper time has elapsed (at timing t5). Upon terminating themeasurement of the frequency of the TXVCO 240 b, the control circuit 260is notified of the termination, so that the control circuit 260 makesthe initial settings for the internal circuits of the high frequency IC200 for a transmission/reception operation after the measurement.

After the initial settings, the baseband circuit 300 supplies the highfrequency IC 200 with “Synthesiser Control Word” which includes a valuethat should be set in the counter 22 (information on the frequency of achannel to be used) (at timing t6). In response, the control circuit 260enters the warm-up mode, selects a band to be used by the RFVCO 250based on the frequency information from the baseband circuit 300 and themeasured frequencies stored in the register 18, and sets a frequencyvalue in the counter 22. Then, the control circuit 260 forces the RFVCO250 to oscillate to bring the reception PLL loop into a locked state.The Synthesiser_Control word includes a single control bit [TR] which isused to tell the IC that the next active slot will be a ‘Transmit’ or a‘Receive’ slot. If ‘Transmit’ is selected, then sending the Synthesiserword into the radio chip turns on the IFVCO and IF synthesiser. TheIFVCO must be running and locked before the transmit slot. If ‘Receive’mode is selected, sending the Synthesiser word into the IC does not turnon the IFVCO/IF synthesiser. Of course, it is possible for the basebandto send ‘Transmit’ mode and then send a Receive word. In this case, whenthe IC receives the Synthesiser_Control word, the IFVCO/IF synthesiserwill become active and lock onto the correct frequency. Then, when theReceiver word is programmed into the IC, the IFVCO/IF synthesiser willturn off automatically.

Subsequently, the baseband circuit 300 sends “Receiver Control Word” tothe high frequency IC 200 for instructing a reception operation (attiming t7). In response, the control circuit 260 also starts the offsetcancel circuit 213 to cancel the input DC offsets of the amplifiers inthe high gain amplification units 220A, 220B. After the DC offsetscancel, the control circuit 260 enters the reception mode, and operatesthe reception related circuit RXC to amplify and demodulate a receptionsignal. The control circuit 260 also switches the switch SW1 and thelike depending on whether the reception signal conforms to GSM orDCS/PCS. The reception mode is executed in time units called a “timeslot” (for example, every 577 μsec), as in the transmission mode.

Upon termination of the reception mode, the baseband circuit 300 sends“Synthesiser Control Word”which includes values that should be set inthe counters 22, 32 (information on the frequencies of used channels)and instructs the warm-up mode (at timing t8). In response, the controlcircuit 260 enters the warm-up mode, selects a band to be used by theRFVCO 250 based on the frequency information from the baseband circuit300 and the measured frequencies stored in the register 18, and setsfrequency values in the counters 22, 32. Then, the control circuit 260forces the RFVCO 250 and IFVCO 230 to oscillate to bring an RFPLL and anIFPLL loop into a locked state.

Subsequently, the baseband circuit 300 sends the high frequency IC 200“Transmitter Control Word” which instructs a transmission operation (attiming t9). In response, the control circuit 260 enters the transmissionmode, selects a band to be used by the TXVCO 240 based on the frequencyinformation from the processing circuit 38 and the measured frequenciesstored in the register 37, operates the TXVCO 240 a or 240 b and thetransmission related circuit TXC, and brings the transmission PLL loopinto a locked state in which a transmission signal is modulated andamplified. The control circuit 260 also turns on the transmission switchSW4, and switches the switch SW2 and the like depending on whether thetransmission signal conforms to GSM or DCS/PCS. Whether to use the TXVCO240 a or 240 b is determined by a predetermined code included in thecommand supplied from the baseband circuit 300.

FIG. 8 shows in greater details the timings at which the frequency ismeasured for each of bands assigned to the RFVCO 250, IFVCO 230, TXVCO240 a, 240 b in the idle mode. In FIG. 8, T0 indicates a period in whichthe frequency is measured for the RFVCO 250; T1 indicates a period inwhich the frequency is measured for the IFVCO 230; T2 indicates a periodin which the frequency is measured for the TXVCO 240 a; and T3 indicatesa period in which the frequency is measured for the TXVCO 240 b.

As shown in FIG. 8, in the high frequency IC of this embodiment, thefrequency is measured for each of the lower 15 bands (0–14) of 16 bandsassigned to the RFVCO 250, while the frequency is measured for each ofthe lower seven bands (0–6) of eight bands assigned to the TXVCOs 240 a,240 b. This is because, even without measured values for the highestfrequency bands, when a band corresponding to a frequency specified bythe baseband circuit does not fall within any of the bands, thefrequencies of which have been measured, the band decision circuit mayselect the highest band, the frequency of which is not measured, or isobliged to select the highest band.

As can be seen from FIG. 8, in the high frequency IC of this embodiment,the frequency of the RFVCO 250 is measured in synchronism with a clockRFCLK, the period of which is 5 μs, while the frequencies of the IFVCO230 and TXVCO 240 a, 240 b is measured in synchronism with a clockIFCLK, the period of which is 1 μs. This is because the measurement ismatched with the operating rate of the counters 22, 32 which operate inthe transmission/reception mode.

Referring next to a flow chart of FIG. 9, description will be made on aprocedure for measuring the frequencies of the IFVCO 230, TXVCO 240 a,240 b using the counter 32 under the control of the control circuit 260.

As VCO Calibration Word is set as a command, the frequency is measuredfor the IFVCO 230 (step S1), and in parallel with the measurement, theloop filter 238 of the transmission TXPLL is applied with and held inthe DC voltage VDC for measuring the frequency (step S2). As a settlingtime has elapsed after the frequency was measured for the IFVCO 230 sothat the voltage applied to the loop filter 238 settles to the DCvoltage VDC (steps S3, S4), the control circuit 260 makes preparationfor measuring the frequency of the TXVCO, such as setting a value in theprescaler 31 (the initial value is “16”), setting the N counter 32N tooperate as an 11-bit counter for the measurement, and the like (stepS5).

Next, the control circuit 260 specifies, using a TXVCO BAND_SELECTregister whether the frequency is measured for the TXVCO 240 a or 240 b(step S6). In this embodiment, the control circuit 260 first specifiesthe TXVCO 240 a for GSM by setting one to BAND_SELECT register. Then,the control circuit 260 specifies a band under measurement using bandthe selection code VB2–VB0 (step S8). Subsequently, the control circuit260 operates the N counter 32N to count, for example, for 40 μs (stepS9). Then, the counted value is stored in the register 37 (step S10).The control circuit 260 determines whether or not the frequency has beenmeasured for the last band, and if not, the control circuit 260 updatesthe band selection code VB2–VB0 to change the band under measurement andreturns to step S7 (steps S11, S12).

On the other hand, if the control circuit 260 determines at step S11that the frequency has been measured for the last band, the flowproceeds to step S13, where the control circuit 260 changes theBAND_SELECT register for specifying the TXVCO under measurement to “2”and returns to step S7. Subsequently, the control circuit 260 executessteps S14–S18 similar to steps S8–S12 to measure the frequency of theTXVCO 240 b, and then returns to the idle mode (step S19).

While the invention made by the inventors has been described in specificmanner with reference to the embodiment, the present invention is notlimited to the foregoing embodiment. For example, in the foregoingembodiment, the frequencies of the TXVCOs 240 a, 240 b are measuredusing the counter 32 provided for the IFVCO 230. Alternatively, themeasurement may be made using the counter 22 provided for the RFVCO 250.Further alternatively, the frequency of the TXVCO 240 a may be measuredusing the counter 32 provided for the IFVCO 230, while the frequency ofthe TXVCO 240 b may be measured using the counter 22 provided for theRFVCO 250. In addition, instead of measuring the frequencies of theTXVCOs 240 a, 240 b for all frequency bands, the frequencies may bemeasured only for odd-numbered bands or for even-numbered bands, andfrequencies of bands, not measured, may be calculated by averagingmeasured frequency values of the respective preceding and subsequentbands.

Further, in the foregoing embodiment, the frequencies of the TXVCOs 240a, 240 b are measured using the counter 32 provided for the IFVCO 230 ina viewpoint of limiting an increase in the circuit scale. Alternatively,an additional counter 32 may be separately provided for measuring thefrequencies of the TXVCOs 240 a, 240 b, such that the frequencies of theTXVCOs 240 a, 240 b may be measured in parallel with measurements of thefrequencies of the RFVCO 250 or IFVCO 230 in response to a singlecommand. Advantageously, the addition of the counter 32 permits thefrequencies of a plurality of VCOs to be measured in a short time at theexpense of a slight increase in the circuit scale.

Also, in the foregoing embodiment, the IFVCO 230 is configured to beoperable in any of four bands, however, the IFVCO may be configured tobe operable in a single band and may not be capable of measuring thefrequency. In this case, the frequencies of the TXVCOs 240 a, 240 b maybe measured using the counter 22 provided for measuring the frequency ofthe RFVCO 250.

Also, the foregoing embodiment has been described for an example inwhich inductors are connected external to the IC, inductors fabricatedon the IC may be used as the inductors L1, L2.

In the foregoing description, the present invention made by theinventors has been discussed in connection with a high frequency IC foruse in a radio communication system such as a portable telephone whichis capable of communicating in accordance with four communicationschemes: GSM850, GSM900, DCS1800, PCS1900 which are the field ofutilization that underlies the invention. The present invention,however, is not limited to this particular high frequency IC, but may beapplied to a transmission VCO mounted in a high frequency IC for use ina portable telephone which can support a communication scheme calledEDGE that has a QPSK modulation mode which is something like acombination of amplitude modulation with phase modulation in GSM. Thehigh frequency IC has a transmission related circuit which employs aso-called polar loop configuration that has a phase loop and anamplitude loop for a modulation method.

This system is not limited to GSM operation, GPRS operation or EDGEoperation or WCDMA operation. This band-selection system for oscillatorsin a radio IC can be used in any communications system where the IC hasmultiple switched band oscillators.

In conclusion, the foregoing embodiment provides the followingadvantages.

The transmission semiconductor integrated circuit described in theembodiment is capable of oscillating over a wide frequency range,completing a selection of a band to be used by a transmission VCO in ashort time, and reducing the burden on a baseband circuit fordetermining a band to be used by the transmission VCO, when anoscillator circuit forming part of a transmission PLL circuit isfabricated on a single semiconductor chip together with other oscillatorcircuits such as an oscillator circuit which forms part of a receptionPLL circuit and an oscillator circuit for an intermediate frequency.

Also, the communication semiconductor integrated circuit in theforegoing embodiment can communicate using signals in a plurality offrequency bands, and comprises oscillator circuits RFVCO, IFVCO, TXVCOwhich can be fabricated on the same semiconductor chip together with amodulator circuit, a demodulator circuit, and the like to thereby reducethe number of parts which make up the system and achieve a reduction insize of the system.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. A communication semiconductor integrated circuit, comprising: afirst, a second, and a third oscillator circuit each capable ofoscillating in a plurality of frequency bands; one or two or morefrequency counters for measuring oscillating frequencies of said first,second, and third oscillator circuits; storage means for storing resultsof measurements on the oscillating frequencies made by said frequencycounter in each of the frequency bands of said first, second, and thirdoscillator circuits; and a control circuit for applying a predeterminedDC control voltage to said first, second, and third oscillator circuitsto oscillate at respective oscillating frequencies, and for controllingsaid frequency counter to measure the oscillating frequencies and storethe measured frequencies in said storage means, said control circuitconfigured to determine operating frequency bands for said first andsecond oscillator circuits based on set frequency information for saidfirst and second oscillator circuits, and the results of measurementsstored in said storage means, and to determine an operating frequencyband for said third oscillator circuit based on the set frequencyinformation for said first and second oscillator circuits, and theresults of measurements stored in said storage means.
 2. A communicationsemiconductor integrated circuit according to claim 1, wherein: saidthird oscillator circuit is configured to oscillate at a frequencycorresponding to a difference in frequency between oscillating signalsrespectively outputted from said first and second oscillator circuits orbetween signals divided from said oscillating signals; and said controlcircuit is configured to calculate a value corresponding to a targetoscillating frequency for said third oscillator circuit from the setfrequency information for said first oscillator circuit and the setinformation for said second oscillator circuit, and to determine theoperating frequency band for said third oscillator circuit based on saidcalculated value and the result of measurement on said third oscillatorcircuit stored in said storage means.
 3. A communication semiconductorintegrated circuit according to claim 2, further comprising: a modulatorcircuit for modulating an oscillating signal of said second oscillatorcircuit or a signal divided from said oscillating signal in accordancewith a baseband signal; a mixer for mixing an oscillating signal fromsaid first oscillator circuit or a signal divided from said oscillatingsignal with a signal in accordance with an oscillating output of saidthird oscillator circuit to generate a frequency at a frequencycorresponding to a difference in frequency between said two signals; anda PLL loop for controlling said mixer such that the frequency of anoutput signal from said mixer matches the frequency of an output signalfrom said modulator circuit.
 4. A communication semiconductor integratedcircuit according to claim 1, further comprising: a first frequencycounter for measuring the oscillating frequency of said first oscillatorcircuit; and a second frequency counter for measuring the oscillatingfrequency of said second oscillator circuit, wherein either said firstfrequency counter or said second frequency counter is configured tomeasure the oscillating frequency of said third oscillator circuit ineach frequency band.
 5. A communication semiconductor integrated circuitaccording to claim 4, wherein said third oscillator circuit includes afirst oscillator to be used for a first frequency band and a secondoscillator to be used for a second frequency band which is differentfrom the first frequency band, and wherein said first oscillator circuitis assigned a number of frequency bands larger than the number offrequency bands assigned to said second circuit and one of said firstand second oscillators included in said third oscillator circuit, andsaid second frequency counter is configured to measure the oscillatingfrequency of said third oscillator circuit.
 6. A communicationsemiconductor integrated circuit according to claim 1, furthercomprising a counter disposed before each said oscillator circuit fordividing an oscillating signal from the oscillator circuit associatedtherewith, wherein said counter functions as said frequency counter. 7.A communication semiconductor integrated circuit according to claim 1,further comprising a demodulator circuit for generating a demodulatedsignal based on the oscillating signal outputted from said firstoscillator circuit or a signal divided from said oscillating signal, anda received signal.
 8. A communication semiconductor integrated circuitaccording to claim 1, wherein said third oscillator circuit includes afirst oscillator for generating a transmission signal conforming to aGSM scheme, and a second oscillator for generating a transmission signalconforming to a DCS scheme, and said third oscillator circuit isconfigured to selectively operate one of said first and secondoscillators corresponding to a frequency band determined by said controlcircuit.
 9. A radio communication system comprising: a communicationsemiconductor integrated circuit according to claim 8; a basebandcircuit for extracting data from a received signal downconverted to adesired frequency by said communication semiconductor integratedcircuit, and converting transmission data to I, Q signals; and saidcontrol circuit configured to receive from said baseband circuit saidset frequency information and an instruction for selecting either saidfirst oscillator or said second oscillator.
 10. A radio communicationsystem according to claim 9, capable of transmitting and receiving inaccordance with three or more communication schemes including at least aGSM scheme, a DCS scheme, and a PCS scheme, wherein said firstoscillator is configured to generate a transmission signal conforming tothe GSM scheme, and said second oscillator is configured to generatetransmission signals conforming to the DCS scheme and PCS scheme.